The present invention relates to a method of making a laminated recorded disc and particularly to a method of making a recorded disc having an outer layer of a conductive plastic around a core of plastic material.
A capacitive video disc system has been described by Clemens in U.S. Pat. No. 3,842,194. The disc described comprises a plastic disc containing an information track having audio, video and color information in the form of a surface relief pattern arranged in the surface of the disc which can be played back with a stylus. The video disc of Clemens had a conductive metal coating to provide the conductivity required for capacitive pickup and a thin layer of a dielectric material thereover. An electrode on the playback stylus completed the capacitor.
Improvements have been made in this system whereby the disc is made from a conductive plastic material, as disclosed by Fox et al in copending application Ser. No. 105,550 filed Dec. 20, 1979, which is a continuation of application Ser. No. 818,279 filed July 25, 1977, now abandoned. A polyvinylchloride based molding composition is mixed with sufficient amounts of finely divided carbon black particles so that the resulting composition has the conductivity required for capacitive playback. A thin layer of the polyvinylchloride surrounds each of the conductive carbon particles so that a thin dielectric layer is present at the surface. The use of a conductive plastic eliminates the need for separate metal and dielectric layers on the disc surface and thus has simplified manufacture of the disc.
The disc described in the Fox et al application is made by forming a preform of the carbon black filled molding composition and placing the preform between the platens of a molding press. The heated platens are closed against the preform with pressure to cause the material of the preform to flow radially outwardly to fill the mold cavity provided by the platens. This method is described in the copending application of M. L. McNeely, Ser. No. 066,769, filed Apr. 30, 1979, entitled "Video Disc Preform and Method of Making a Disc Therefrom". The preform can be formed in a mold which automatically transfers the preform to the disc molding apparatus. Such a mold is shown in copending application of C. B. Carroll et al, Ser. No. 102,620, filed Dec. 11, 1979, now U.S. Pat. No. 4,281,816 entitled "Automatic Apparatus for Molding a Preform".
In the recorded disc disclosed in the Fox et al application although the carbon black particles are only required at the surfaces of the disc so as to provide electrically conductive surfaces, the carbon black particles extend completely throughout the entire volume of the disc. It would be desirable to limit the carbon black particles to the surfaces of the discs which would reduce the amount of carbon black used to make the disc and thereby reduce the cost of the disc.
Recorded discs having surface layers of a material different from the core of the disc are known and there are various techniques for making such laminated discs. One technique is to place a plurality of sheets, e.g. a sheet of the core material between two sheets of the outer material, between two platens of the mold. The heated platens are closed against the laminated sheets to bond the sheets together and impress the surface relief pattern into the surface layers. U.S. Pat. Nos. 3,247,298 to H. Utiger, issued Apr. 19, 1966, entitled "Method for the Production of Sound Records" and No. 3,584,094 to J. Rock et al, issued June 8, 1971, entitled "Method of Making Picture-Type Phonograph Records" illustrate this technique. Another technique is to place a preform of the core material between two sheets of the outer material between the mold platens. This is shown in U.S. Pat. No. 2,452,821 to L. A. Wood, Jr., issued Nov. 2, 1948, entitled "Method of Making Phonograph Records". Still another technique is to use a sheet of the core material between one or more preforms of the outer material. This is shown in U.S. Pat. Nos. 4,213,927 to J. D. Alberti, issued July 22, 1980, entitled "Method of Making Picture Phonograph Record" and 4,248,818 to K. Elizabeth, issued Feb. 3, 1981, entitled "Process for Making a Record".
A problem with each of the above techniques for making a laminated disc is that they require the handling of a plurality of parts, such as a plurality of sheets, or a sheet and one or more preforms. Therefore, it would be desirable to be able to make a laminated disc having a conductive outer layer surrounding a core material using a single preform of the type shown in the previously referred to McNeely application.